This invention relates to a system for storing and releasing energy as heat and to a process for operating such a system.
It has been known for some time that hydrated salts having a heat of fusion of above about 116 kJ/kg (50 BTUs per pound) can be used as heat storage media. Heat is absorbed in these hydrated salts causing a phase change from solid to liquid. Thereafter, the heat is recovered by allowing the molten salt to solidify and release the heat which was absorbed.
Certain serious problems have heretofore been encountered in the operation of these hydrated salt heat storing systems. The phenomenon known as supercooling prevents the release of useful heat from these systems. This occurs when a molten salt cools below its freezing point without the formation of solid material. Thus, instead of employing the heat of fusion of the hydrated salt, only the sensible heat, which is many times less than the heat of fusion, is recovered. The prior art has attempted to avoid the problem of supercooling by introducing nucleating agents which provide an initial seed for the formation of salt crystals.
An additional problem associated with the use of hydrated salts as bulk heat storage media is the known tendency of these materials to also form other salts of lower hydration with a corresponding loss in the heat of fusion. Since the density of these crystals is greater than the density of the solution the crystals settle to the bottom of the container. During subsequent heat absorption-heat releasing (melting-solidification) cycles the system becomes stratified vertically into layers with both decreasing density and composition. Eventually the amount of lower hydrated salt, or even anhydrous salt, grows progressively larger with a corresponding increase in an upper weak liquor layer and a significant deterioration in the energy storage capability of the system. Prior art attempts to overcome this stratification problem have involved the use of thickeners to prevent the formation of layers by so dispersing the salt media and excess water in such a way that the appropriate number of water molecules are available in the immediate vicinity of the anhydrous or lower hydrated salt in order to recombine in the next heating cycle.
It has also been suggested by the prior art that both problems, i.e., supercooling and stratification, could be avoided by mixing or stirring of the molten hydrated salt. However, such a solution was discarded due to the inconvenience or impossibility of affecting agitation in heat storage systems. Indeed, much of the focus of the prior art workers in this field have been on the use of a large number of relatively small sealed containers of hydrated salts having at least one dimension thin enough to assist in preventing stratification. Generally, the prior art teaches away from the use of bulk containers of hydrated salts as heat storage media and those prior art systems which do contain the salt in bulk are awkward to operate and also require nucleating agents to prevent supercooling.
Ideally, heat storage systems for large buildings or homes, e.g., in conjunction with solar heat receiving means, would involve storage of the hydrated salts in bulk. Such a system has the obvious advantage of lower cost, maintenance and the like.
It is accordingly an object of this invention to provide a process for storing and releasing heat employing the heat of fusion of hydrated salts contained within bulk containers.
It is another object of this invention to provide a heat storage-releasing process employing such salts stored in bulk and which avoids the problems of supercooling and stratification.
It is an additional object of this invention to provide a system for storing and releasing heat in a hydrated salt stored in bulk.
These and other objects will become apparent from the description which follows.